Voltage buffers (such as emitter followers or source followers) are commonly used as an isolation buffer between upstream and downstream circuits. For example, voltage buffers may be used in sampler circuits to maintain good linearity of the output signal by isolating the input from the switching effects of the sampling switches. In theory, an ideal voltage buffer has characteristics of an infinite input impedance and zero output impedance over an infinite bandwidth to drive the downstream circuit. Thus, the voltage buffer may supply a load device a load current while keeping the output voltage fixed because of the very low output impedance.
However, in practice, voltage buffers may not have the perfect characteristics of an ideal voltage buffer due to various factors. For example, FIG. 1 shows an emitter follower 100 (the following discussion is similarly applicable to source followers). In the emitter follower, a transistor device 106 is connected to a bias current source 108 that supplies substantially constant current I and to a load device ZL 110. The output Vout of the emitter follower is designed to follow the input Vin 104 (possibly with an offset) when the output impedance from the transistor is small. In practice, transistor devices do not have the perfect characteristics as described above. Instead, the transfer function of the transistor 106 may have nonlinear components that may cause distortions in the output Vout and thus degrade the voltage buffer's performance.
When the input signals include large AC components, the buffer bias current (i.e., current from the emitter to the collector of the transistor) may also vary from I, which may further cause variations in the current passing through the load devices ZL 110. The variations in the load current further may cause changes in parameters (such as transconductance) of the load device 110 and hence induce further nonlinearity in the voltage buffer output.
Conventionally, to reduce the nonlinearity, voltage buffers have been designed to have a bias current I significantly larger than the variable signal current supplied from the buffer to the load device. This may decrease the relative effect of the current variations and hence improve the buffer's linearity. However, a large bias current is associated with large DC power consumptions. Moreover, when the load device is capacitive in nature (such as a sample-and hold sampler circuit as shown in FIG. 2), the current supplied to the load device may increase with the signal frequency. Thus, the linearity of the buffer may suffer considerably at high frequencies despite a high bias current.
U.S. Pat. No. 6,778,013 (the '013 patent) and U.S. Pat. No. 7,119,584 (the '584 patent) to the same inventor describe a class of voltage buffers that use replica current generators generating replica load currents to compensate the current variations in the signal path of voltage buffers and thus counter the nonlinearity at the buffer output. The replica current generators disclosed in the '013 and '584 patents may include complex circuits that include an additional follower and at least one additional replica current source. Additionally, the complex replica generator circuits disclosed the '013 and '584 patents are slow to switch and consume additional powers.
Therefore, there is a need for an efficient solution to reduce the nonlinearity at the output of voltage buffers.